Downhole drilling motors have been used for many years in the drilling of oil and gas wells and other wells. In the usual mode of operation, the rotational power output shaft of the motor and the drill bit will rotate with respect to the housing of the motor. The housing, in turn, is connected to a conventional drill string composed of drill collars and sections of drill pipe. This drill string extends to the surface. Drilling fluid is pumped down through the drill string to the bottom of the hole and back up the annulus between the drill string and the wall of the bore hole. The drilling fluid cools the drill bit and removes the cuttings resulting from the drilling operation. In the instances where the downhole drilling motor is a hydraulic powered type, such as a positive displacement type motor, the drilling fluid also supplies the hydraulic power to operate the motor. See FIG. 1.
Virtually all downhole drilling motors have three basic components:
1. Motor section
2. Vertical thrust bearings
3. Radial bearings
The bearings can be placed in a separate package or unit at the motor section and thus can be used on any type of motor (i.e., turbodrills, positive displacement motors, etc.).
There are two basic types of downhole drilling motors:
1. Turbodrills
2. Positive displacement motors.
Turbodrills utilize the momentum change of drilling fluid (i.e., mud) passing through curved turbine blades to provide power to turn the bit. Turbodrills turn at speeds of 600 to 3,000 rpm. Positive displacement motors have fixed volumetric displacement and their speed is directly proportional to the flow rate of the hydraulic power fluid.
There are two basic types of positive displacement motors in use:
1. Moineau motors have a helical rotor within the cavity of a stator which is connected to the housing of the motor. As the drilling fluid is pumped down through the motor, the fluid rotates the rotor.
2. Vane motors have large volumetric displacement and therefore deliver higher torques at lower speeds.
Thrust bearing failure in downhole motors is a problem because of high dynamic loads produced by the action of the bits and by drill string vibrations. One major oil company placed a recorder at the hole bottom and found that dynamic loads were often 50% higher than the applied bit weight. It was found on occasion that the bit bounced off the bottom and produced loads in excess of 120,000 pounds when drilling at an applied bit weight of 40,000 pounds. See discussion in U.S. Pat. No. 4,246,976, incorporated by reference. These high loads can cause rapid failure of the thrust bearings and bearing mandrels; consequently, these bearings must be greatly over-designed to operate in the hostile downhole environment.
At least three types of thrust bearings have been used in downhole drilling motors:
1. Rubber friction bearings
2. Ball or roller bearings
3. PDC Diamond Bearings.
Radial bearings are required between the bearing housing and the rotating mandrel transmitting power from the motor power output to the bit. Radial bearings are usually subjected to lower loads than the thrust bearings and therefore have much longer life. The basic types of radial bearings used in downhole motors are:
1. Marine bearings
2. Roller or ball bearings
3. Metal to metal carbide bearings.
Most motors contain metal to metal radial bearings. These bearings are frequently lubricated by circulating mud through them. However, some bearing systems are sealed and are lubricated using lubricant (grease/oil) injected into the bearing by a hydraulic piston assembly.
For a further discussion of downhole drilling motors and their operations, see U.S. Pat. Nos. 3,840,080; 4,246,976; 4,492,276; 5,495,900; 5,090,497; 6,183,226; 6,905,319 and Canadian Patent No. 2,058,080, incorporated by reference.